The present invention concerns an SCR (selective catalytic reduction) exhaust aftertreatment device for an internal combustion engine in accordance with the features in the preamble of claim 1.
In motor vehicles, internal combustion engines are used to convert the chemical energy contained in fuel into mechanical energy to power the motor vehicle via the combustion process. In the process, due to the ideal Carnot cycle, about 40% of the energy contained in the fuel is converted to mechanical energy. The rest of the energy contained in the fuel is converted to heat, and exhaust gases arise as a result of the combustion process. Among other things, these exhaust gases contain nitrogen oxides (NO, NO2).
In recent years, in particular also in automotive manufacturing, Selective Catalytic Reduction (SCR), in which a reductant is added to the exhaust and nitrogen oxides are selectively reduced and undesired side reactions such as the oxidation of sulfur dioxide to sulfur trioxide are suppressed, has established itself in the state of the art.
Ammonia is typically used as the reductant. In automotive engineering this is supplied in the form of an aqueous urea solution and is known, among other things, under the registered trademark AdBlue. One objective in particular is the reduction of nitrogen oxide emissions in diesel engines by means of Selective Catalytic Reduction, which is also referred to as urea injection.
The reductant added to the exhaust stream has to be mixed with the exhaust stream. The reductant itself is often injected into the exhaust stream with an injector. Typical injectors generate pronounced filament jets with relatively large and stable individual droplets.
To now achieve good mixing of the exhaust stream and the injected reductant, mixer elements, which create a swirl to bring about relative motion between the exhaust stream and the reductant, are known from the state of the art. This in particular extends the droplet trajectory of the injected reductant, thus increasing the evaporation tendency and resulting in better mixing.
A mixing unit, in which a first swirl component is followed by a downstream counterswirl component, is known from DE 10 2012 224 198 A1 for example. At the same time, this also increases the exhaust gas back pressure. This elevation of the exhaust gas back pressure is undesirable, however, because it has a negative effect on the efficiency of the internal combustion engine.
After the exhaust stream is mixed with the reductant, the exhaust stream enters a downstream SCR catalytic converter. In most cases an additional oxidation catalyst is positioned upstream of the location at which the reductant is injected into the exhaust stream.